1. Field of the Invention
The present invention relates to a low-noise optical frequency converter that is able to convert light having a reference frequency to light having a controlled frequency difference.
2. Description of the Prior Art
Optical frequency converters use a number of ways to convert the frequency of input light, including (1) a method using non-linear optical crystal to mix two light inputs that is a well-known method also used for doubling laser frequencies; (2) a method using a mode-locked laser that comprises using an optical modulator, isolator and Fabry-Perot etalon provided in a laser resonator to generate optical pulses, which method is also used to generate a sideband of a frequency fp that is Km times higher than the phase modulation frequency fm (fp=Km×fm); and (3) a method modulating the light with a microwave signal to derive a sideband that is used to convert the optical frequency.
The above method (3) can be further subdivided into (3-1) a method in which the input light is intensity-modulated or phase-modulated by a microwave signal and the target sideband is selectively extracted by using a filter or the like, to obtain light having a frequency that is the sum or difference of the light frequency and the frequency of the microwave signal, and whose merit is the ability to select just the target sideband and whose drawback is that it requires a filter, and in cases where the microwave signal is to be changed, the filter characteristics have to be adjusted accordingly; and (3-2) a method in which input light is phase-modulated by a microwave signal, an optical circuit is used to cancel the optical carrier and a sideband on one side, and the other sideband is extracted to convert the optical frequency and which method is known as single sideband (SSB) modulation and does not use a filter, so there is no need to adjust a filter even when the microwave signal is to be changed though a drawback is that the obtained optical signal contains order sidebands.
The present invention relates to the method (3-2) mentioned above and will be described below.
Optical SSB modulation is described in detail in, for example, Reference 1 (Shimotsu, Izutsu, et al., “LiNbO3 optical SSB modulator for next-generation communications,” Optical Alliance, vol. 27, July 2000) and Reference 2 (Higuma, et al., “Development of optical frequency shifter/SSB-SC modulator using X-cut LiNbO3,” Technical Report of IEICE, OPE 2001-159 (2002-2)).
In particular, Reference 2 describes modulation using an SSB modulator constituted by the optical frequency shifter using X-cut LiNbO3 shown in FIG. 1. With reference to FIG. 1, an optical carrier is input and a modulated light wave is output, using a modulator 1. The modulator 1 comprises a Mach-Zehnder interferometer (MZC) type SSB modulator that uses first and second Mach-Zehnder interferometer type phase modulators (MZA) and (MZB) provided on respective optical paths to modulate the above optical carrier. Here, it is assumed that the incident light is exp(jωt) when ω is the angular frequency of the incident light, Ω is the angular frequency of the microwave signal and Ø is the degree of modulation, and for the sake of the explanation it is assumed that the modulating microwave signal is a monotone signal Ø sin(Ωt). This signal is applied to RFA of the modulator 1 and signal Ø cos(Ωt) is applied to RFB of the modulator 1. This signal is the above signal phase shifted by π/2 (90 degrees), and therefore can be generated from the same signal by using a phase shifter. Also, a bias voltage is applied to DCC, to apply a phase difference of π/2 (90 degrees) between light waves transmitted by the two arms of MZC. Simultaneously, a bias voltage is applied to DCA and DCB, applying a phase difference π (180 degrees) between light waves transmitted by the respective arms of MZA and MZB, suppressing the carrier. That is, the light waves passing through the MZA are subjected to electric fields having mutually opposed orientations and are given a phase differential, so that the output from the MZA is as follows.fA(t)=exp(jωt){exp(jØ sin(Ωt))+exp(−jØ sin(Ωt))exp(jπ)}  (1)And, the light waves passing through the MZB are as follows.fB(t)=exp(jωt){exp(jØ cos(Ωt))+exp(−jØ cos(Ωt))exp(jπ)}  (2)Therefore, a light wave output from MZC will be
                                          f            C                    ⁡                      (            t            )                          =                              exp            ⁡                          (                              j                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                t                            )                                ⁡                      [                                                                                                      {                                                                        exp                          ⁡                                                      (                                                          j                              ⁢                                                                                                                          ⁢                                                              ϕsin                                ⁡                                                                  (                                                                      Ω                                    ⁢                                                                                                                                                  ⁢                                    t                                                                    )                                                                                                                      )                                                                          +                                                                              exp                            ⁡                                                          (                                                                                                -                                  j                                                                ⁢                                                                                                                                  ⁢                                                                  ϕsin                                  ⁡                                                                      (                                                                          Ω                                      ⁢                                                                                                                                                          ⁢                                      t                                                                        )                                                                                                                              )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              j                                ⁢                                                                                                                                  ⁢                                π                                                            )                                                                                                                          }                                        +                                                                                                                                                                                            exp                          ⁡                                                      (                                                          j                              ⁢                                                                                                                          ⁢                                                              ϕcos                                ⁡                                                                  (                                                                      Ω                                    ⁢                                                                                                                                                  ⁢                                    t                                                                    )                                                                                                                      )                                                                          +                                                                              exp                            ⁡                                                          (                                                                                                -                                  j                                                                ⁢                                                                                                                                  ⁢                                                                  ϕcos                                  ⁡                                                                      (                                                                          Ω                                      ⁢                                                                                                                                                          ⁢                                      t                                                                        )                                                                                                                              )                                                                                ⁢                                                      exp                            ⁡                                                          (                                                              j                                ⁢                                                                                                                                  ⁢                                π                                                            )                                                                                                                          }                                        ⁢                                          exp                      ⁡                                              (                                                  j                          ⁢                                                                                                          ⁢                                                      π                            /                            2                                                                          )                                                                                                                  ]                                              (        3        )            Ignoring fifth-orders and higher as being very minor, we get equation (4).fC(t)=exp(jωt){J+3(Ø)exp(j3Ωt)+J−1(Ø)exp(−jΩt)}  (4)From equation (4), it can be seen that a first-order lower-sideband and a third-order upper-sideband are output. It is well known that the modulation index Ø can be decreased to damp higher-order terms. Also, estimating as in the above, applying a phase difference of −π/2 (−90 degrees) between the light waves traveling through the two arms of MZC makes it possible to derive a first-order upper-sideband and a third-order lower-sideband.
As described above, with an optical frequency converter using conventional optical SSB modulation, the drawback is that output of a first-order sideband is accompanied by a third-order sideband. Moreover, a higher modulation index Ø gives rise to the output of larger sidebands corresponding to higher-order odd-numbered terms.
In the above explanation, Ø sin(Ωt) is applied to RFA, and Ø cos(Ωt) is applied to RFB having a phase difference of 90 degrees compared to Ø sin(Ωt). If the phase difference deviates from 90 degrees, the term cancelled in equation (3) will not be cancelled, and third- and higher-order terms will increase, increasing the noise component.
An optical frequency converter that uses optical SSB modulation has the merit of not having to adjust a filter when the microwave signal is changed, since the converter does not use a filter. However, a drawback is that the converted-optical-frequency optical signal that is obtained includes third- and higher-order odd-numbered sidebands.
With respect to a frequency converter that uses optical SSB modulation, the present invention achieves a low-noise optical frequency converter that makes it possible to obtain an optical signal in which third-order sidebands, which among third- and higher-order odd-numbered sidebands have the highest signal strength, are suppressed.
In the case of optical frequency conversion using optical SSB modulation, it is necessary to prepare modulation signals having a 90-degree phase difference. Deviation from this 90-degree phase difference increases the noise component. Thus, noise can also be reduced if such deviation is suppressed.
Moreover, there is the merit that there is no need to adjust a filter when the microwave modulation signal is changed, since there is no filter. And there is the drawback that the converted-optical-frequency optical signal that is obtained includes fifth- and higher-order odd-numbered sidebands. Low-noise conversion can be achieved by suppressing these sidebands.
The present invention was accomplished in the light of the foregoing circumstances, and has as its object to provide a low-noise optical frequency converter that uses optical SSB modulation but is able to suppress noise components even when there is a deviation from the 90-degree phase difference between the two signals used for the modulation, and is able to suppress the third-order sidebands that have the highest signal strength among third- and higher-order odd-numbered sidebands.